Applanating tonometers

ABSTRACT

There is provided a tonometer including an applanating element having a light-transmitting contact face engageable against the eye of a subject for projecting a light beam onto the eye and passing reflected light therefrom. There are also means responsive to the reflected illumination and means responsive to the force of engagement of the applanating element on the eye for obtaining data of changing values of reflected illumination and force as the element is applied to the eye. The contact face has an area larger than a predetermined area of engagement at which the applanting force is to be determined. The instant of applanation of the predetermined area is obtained by interpolation. The cornea is applanated over the contact face area, dependent upon the ratio of the predetermined and larger areas of applanation, in order to determine the force measured at the instant of applanation of the predetermined area. Preferably, means are provided for processing progressive measurements of the force and reflected illumination to derive a measure of acceleration of the applanating element at the moment of applanation, and preferably automatically correcting the applanating force measurement accordingly, in order to compensate for dynamic force components that may appear in the measurement of the force on the applanting element. The readings obtained can thereby be rendered independently, or at least less dependent, of any variations in the rate of application of the contact face against the cornea. In this way the use of the tonometer as a hand-held instrument can be facilitated without compromising the accuracy of measurement.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/GB98/02505 which has an Internationalfiling date of Aug. 20, 1998, which designated the United States ofAmerica.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to applanating tonometers for the measurement ofintraocular pressure (IOP).

2. Description of Related Art

Known applanating tonometers employ a transparent applanating elementwith a contact face through which light can be transmitted onto, and bereflected from, the eye of a subject in a manner which varies with thedegree of applanation. The aim is to determine the force required to putthe contact face in contact with the cornea of the eye to applanate agiven area and so provide a measure of the IOP.

One example of such a tonometer is described in GB 862920, which is aversion of the instrument known as the Goldman tonometer. In order tostandardize the area of applanation and avoid the need forpre-calibration, which-brings its own problems as explained in thatdocument, the eye is observed through an applanating element thatcomprises a split prism. When the element is applied against the eye,the area of applanation can be observed through the split prism as twosemi-circular images displaced relative to each other by an amountdetermined by the parameters of the prism. As the force of applanationand the applanated area increases, the images increase in size and theobject observed when the half images come together, are seen as acontinuous S-shaped line, when the image diameter equals thedisplacement of the images. This dimension is standardized, forpractical reasons, at 3.06 mm.

However, the images produced do not allow a precise and unambiguousdetermination of the coming together of the images. A degree ofsubjective judgement is needed and there can therefore be significantvariations in the readings taken. The readings are also dependent on theskill of the user, because it is not easy to ensure that the applanatingelement is always applied squarely to the cornea. Any tilt of thecontact face relative to the eye will introduce measurement errors.

It would therefore be desirable to provide an instrument that could beoperated with less dependence on the user's skills and judgement inorder to provide a more objective reading of intraocular pressure.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided atonometer comprising an applanating element having a light-transmittingcontact face engageable against the eye of a subject for projecting alight beam onto the eye and passing reflected light therefrom. There arealso means responsive to the reflected illumination and means responsiveto the force of engagement of the applanating element on the eye forobtaining data of changing values of reflected illumination and force asthe element is applied to the eye. The contact face has an area largerthan a predetermined area of engagement at which the applanting force isto be determined. The instant of applanation of the predetermined areais obtained by interpolation. The cornea is applanated over the contactface area, dependent upon the ratio of the predetermined and largerareas of applanation, in order to determine the force measured at theinstant of applanation of the predetermined area.

Preferably, means are provided for processing progressive measurementsof the force and reflected illumination to derive a measure ofacceleration of the applanating element at the moment of applanation,and preferably automatically correcting the applanating forcemeasurement accordingly, in order to compensate for dynamic forcecomponents that may appear in the measurement of the force on theapplanting element. The readings obtained can thereby be renderedindependently, or at least less dependent, of any variations in the rateof application of the contact face against the cornea. In this way theuse of the tonometer as a hand-held instrument can be facilitatedwithout compromising the accuracy of measurement.

In a further aspect, the invention also provides a method of measuringintraocular pressure in which a contact face of an applanating elementis applied against the eye of a subject and measurements are made thatare indicative of both the degree of applanation and the force appliedto the applanating element. The measurements are made while increasingthe pressure of application until the contact face is fully appliedagainst the eye to produce a predetermined lesser extent of applanationby the element by interpolation of the measurements of force.

According to another aspect of the invention, there is provided atonometer comprising an applanating element having a contact faceengageable against the eye of a subject and having means for determiningthe alignment of the contact face to the eye. The means comprising alight source directing a beam onto a beam-splitting element whichtransmits a first part of the beam onto a first reflecting element andreflects a second part of the beam onto a second reflecting element.Light from the first part of the beam is arranged to be reflected by thebeam splitting element to the contact face and light from the secondpart of the beam being arranged to be transmitted through thebeam-splitting element to the contact face. The positions of the imagesof the light from the first and second parts of the beam falling on theretina of the eye and are thereby dependent upon the relative alignmentbetween an optical axis of the tonometer and the optical axis of theeye.

In such a tonometer, it can be arranged that the two reflecting elementsare provided by front faces of a light-emitting device for producing anapplanation measurement beam and a light-receiving device for reflectedillumination from the eye of the measurement beam to derive a measure ofthe applanation pressure.

Conveniently, the elements can provide conjugate paths for the lighttransmitted for the light source to their respective reflectingsurfaces. If the reflecting surfaces are located symmetrically to theoptical axis of the tonometer, it can be arranged that by bringing theobserved images into coincidence with each other will indicate thatalignment has been achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

One embodiment of the invention will be described by way of example withreference to the accompanying drawings, in which:

FIGS. 1 and 2 are plan and front views of an applanating tonometeraccording to the invention with the cover of its housing removed;

FIG. 3 is a plan view of the electro-optical unit of the instrument to alarger scale;

FIG. 4 is a graph illustrating the operation of the instrument of FIGS.1-3; and

FIG. 5 is a flow diagram illustrating the sequence of operations inperforming a reading.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring first to FIGS. 1 and 2, mounted in a housing 2 of theinstrument, which can be held in the hand of the operator, is anelectro-optical unit 4 comprising an applanating probe 6 projecting fromthe front wall 8 of the housing. A circuit board 10 in the housingcarries a microprocessor μP connected to electrical elements of the unit4 for evaluating the signals they produce.

The electro-optical unit 4 comprises a base plate 12 on which aresecured a U-frame 14 (FIG. 3) and a front wall 15. In the central limbof the U-frame 14 an infra red emitter 16 is held in a bore 18, alignedon a main central optical axis A—A of the instrument. In coaxial bores20,22 in the side limbs of the U-frame 14 and aligned on an optical axisB—B perpendicular to and intersecting the main optical axis A—A, are agreen LED 24 and a photo-transistor 26. The preferred photo-transistor,Siemens SFH300FA-3, has a very narrow infra-red wavelength receivinghand and a black face which comprises a daylight filter. Located betweenthe arms of the U-frame is a beam splitter 30 having a reflection plane30 a at 45° to both optical axes A—A and B—B, and meeting both axes attheir intersection. The emitter 16 and photo-transistor 26 areequidistant from that point of intersection.

Projections 36 at opposite ends of the front wall 15 support aresiliently, flexible beam 38 having central opening 40 in which theapplanating probe 6 is secured. The probe 6 has a cylindrical body 42aligned with the main optical axis A—A. The body 42 carries anapplanating head 44 closed at its forward end by an optically flatwindow 46 providing the applanating surface to be applied to the corneaC of the eye E of a subject. The probe 6 is maintained in alignment withthe optical axis A—A by a guide 48 (FIG. 1), secured to the front wall 8of the housing, comprising a tubular portion 50 in which the cylindricalbody 42 of the probe is a sliding fit. Fixed to the rear face of thebeam 38 to each side of the probe are strain gauges 52 which respond tothe beam deflection produced when the probe 6 glides in its guide 48.

For reasons of hygiene, the applanating head 44 is readily replaceable.It comprises a skirt 54 with three symmetrically spaced slits 56 toprovide a degree of resilience so that it can be gripped frictionally bythe body 42 and slid out when it is to be replaced.

In use, light from the infra red emitter 16 passing along the mainoptical axis A—A through the beam splitter 30 is directed through thewindow 46 onto the cornea C of the subject. Part of the light reflectedback from the cornea through the window 46 is reflected by the beamsplitter 30 onto the photo-transistor 26 to generate a signal that isdependent upon the intensity of the reflected light. No reflection willoccur where the window is in contact with the cornea, so thephoto-transistor signal is dependent upon the degree of applanation ofthe cornea by the window. Green light from the LED 24 is alsotransmitted through the beam splitter 30 to the phototransistor 26 butthe light is outside the receiving band of that element so that itsoutput is not affected. The LED output serves solely to assist alignmentof the instrument, as will be described further below.

When the applanating probe 6 is forced into contact with the cornea, theflexible beam 38 will be deflected. The strain gauges 52 mounted on thebeam are connected into a bridge circuit (not shown) on the circuitboard 10 to measure the deflection in order to obtain an indication ofthe force applied to the beam. At the initial stage of contact, when theforce is small, there will only be a small interface formed between thewindow 46 and the cornea C. As the force and the area of contactincrease, so the amount of reflected light falls. While the instrumentis applied to the eye the signals from the photo-transistor 26 and thestrain gauges 52 are inputted to a memory store 62 (FIG. 1) connected tothe micro-processor μP so that data can be held of the increasingapplanating force and the reducing amount of reflected light related toa common time base.

The graph of FIG. 4 shows, against the time base, traces a and brespectively of the optical signal and force data as the probe isapplied to the eye and then withdrawn again. For convenience the opticaltrace a is shown inverted and it indicates how, as the probe is appliedwith increasing force, from an initial value S, the light is reflectedfrom the cornea drops with increasing area of contact, and thenincreases again when the force is removed. Once the entire area of thewindow is covered the reflection signal remains constant at a plateauvalue P, even though FIG. 4 shows the force trace b continuing toincrease as the probe may be pressed further against the eye to ensurefull contact has been made. Similarly, as the probe is withdrawn, onlyafter the deflection force has decreased sufficiently for the window tobegin to be uncovered, does the optical reflection signal change.

As described earlier, in conventional applanating tonometers, such asthe Goldman tonometer, the aim is to measure the force at which thecornea is flattened over a predetermined area of 3.06 mm. The diameterof the probe window in the present instrument is made slightly larger at3.3 mm, although the instrument is intended to provide a force readingfor the standard 3.06 mm diameter value, for a reason that will now beexplained.

Referring to the graph of FIG. 4, as can be seen from the optical trace,the signal changes rapidly as the applanating pressure of the probe isincreased, but this change slows progressively in a transition regionimmediately before the window 46 completely covers the cornea and aplateau value is reached. It is not possible, therefore, to identifyprecisely the point at which the plateau value is first reached.Furthermore, in these conditions any noise in the signal represents asource of further uncertainty.

However, it will be seen that the plateau value itself is very clearlyindicated and it is known that this corresponds to a state of fullcontact between the window 46 and the cornea C. By taking the ratio ofthe squares of the actual and standard window diameters, in this examplegiving the value 0.86, it is therefore possible to identify relativelyaccurately a point on the optical trace where the change of reflectivityhas reached 0.86 of the change to the plateau value. This represents thepoint at which there is contact with an area of 3.06 mm diameter and theforce value at the same time instant therefore indicates the straingauge response when the standard area of applanation has been achievedby the probe window.

It may be noted that this procedure gives a reading which is notdependent upon the absolute values of the initial optical signal beforecontact with the cornea nor the plateau signal. Therefore, although thereflectivity of the eyes of different subjects can vary the derivationof the required signal value will not be affected as this is simply anobjective measure of the overall change of signal in the ratio of theactual window size to the chosen standard window size.

A program store 64 (FIG. 1) in conjunction with the microprocessor APexecutes the measurement program described. FIG. 1 also shows a powersupply 6G for the electrical circuitry and read-out device 68 for thevalues obtained.

The manner in which the data of the traces is evaluated in themicroprocessor, will be described with reference to FIG. 5. The storeddata is first smoothed and the start point I (FIG. 4) of the opticaltrace, at which the applanating element makes initial contact with thesubject's eye is determined. The corresponding optical signal value S isextracted and also the plateau value P of the optical trace at aninstant II some time after the transition (indicated at III) to thesteady state optical signal. The difference between the optical signalsat I and II is then evaluated and provides a measure of the total changeof optical signal during the applanation of the cornea to the full 3.3mm diameter of the window, i.e. the dimension H. Taking then a rise ofoptical signal of 0.86H from the value at the start point I, a referencepoint is obtained on the optical trace which identifies the time instantIV, corresponding to the instant of contact with a standard 3.06 mmdiameter window area. The value of the force trace at time instant IVtherefore indicates the force measured by the strain gauges when theapplanation has reached the standard 3.06 mm diameter area, and thatvalue can be displayed on the read-out device 68.

FIG. 5 illustrates some further processing of the force value obtainedto increase the accuracy of measurement. Because the probe is stillmoving forwards at the time instant IV, the strain gauge signal can alsoinclude a dynamic component from any acceleration or deceleration of theprobe. It is therefore desirable to adjust the signal value tocompensate for this effect. The time taken for the movement from initialcontact to applanation of the standard 3.06 mm diameter is given by theinterval between the points I and IV on the optical trace. the distancemoved can be calculated with sufficient accuracy by assuming a sphericalradius of 7.9 mm, for the cornea, to give a value of 0.15 mm. From theoptical trace it is then possible to determine what, if any,acceleration or deceleration is occurring at the time instant IV. Themass acted upon is sufficiently closely represented by the mass of theslidable probe, so on the basis of this data the microprocessor is ableto calculate the acceleration force and compensate the strain gaugesignal accordingly to adjust the value displayed on the read-out device.

When measuring the IOP of a subject, the value obtained will vary, forexample due to variations in instantaneous blood pressure. It isdesirable, therefore, to cross-reference a series of readings in orderto obtain a final read-out that is at least partially compensated forsuch effects. The preferred method of cross-referencing uses fivesuccessive readings, but those giving the highest and lowest IOP valuesare discarded. From the remaining three readings the change of opticalsignal, i.e. the values H, are averaged. Larger H values are found togive larger force measurements and the force values are adjusted toaccord with the mean H value before they are averaged for the read-outof IOP on the read-out device 68.

The instrument described can be arranged as a bench-mounted instrumentwith a mechanical drive for advancing the probe to give a uniformoperating sequence, in which case any correction applied foracceleration force can be a constant value.

If the apparatus is in the form of a hand-held instrument, the operatingprogram is preferably adapted to detect abnormal modes of operation thatmay then occur. In particular, the operator may stop moving the probeforwards too soon to establish a true plateau value or the contact withthe eye may not be maintained for a long enough period. If either faultis detected, the micro-processor can be programmed to abort thefollowing sequence of operations and display an invalid reading message.

A possible source of error arises in the use of an applanating tonometerif the contact face of the probe is tilted in relation to the eye. Thegreen light emitted from LED 24 is used to assist alignment of theinstrument with the eye and so avoid any such error. Some of the lightemission from the LED passes through the reflection plane 30 a of thebeam splitter to impinge on the photo transistor 26 which is aconventional device with a generally hemispherical reflective frontface, so that some light is returned to the beam splitter where a partis reflected at the plane 30 a to pass through the probe 6 onto the eye.Another part of the incident light from the LED 24 is reflected by thebeam splitter 10 onto the infra red emitter 18 which similarly has agenerally hemispherical reflective front face. Part of the lightreflected from the emitter front face will thus be transmitted to theeye is through the reflection plane of the beam splitter and the probe6.

The eye of the subject therefore receives green light reflected onconjugate paths from both the infra red emitter and, to a lesser extentbecause of its black surface, the photo-transistor. In each case, theimage projected by the curved reflecting faces has a central brighterzone merging into a dimmer halo. If the axis of the eye and the axis ofthe probe are in alignment, the images are superimposed directly ontoeach other and a central circular bright spot will be seen by thesubject.

If the axes are tilted or offset relative to each other, there will be achange in the images seen because the reflected light distributionreaching the eye of the subject is no longer symmetrical about a centralaxis. The subject will therefore be able to indicate that the probe iscorrectly aligned both before and during operation of the instrument byobserving when a symmetrical brighter zone lies in the center of thesurrounding halo.

It will be understood that this method of monitoring alignment can beused independently of whether the method of measuring IOP describedabove is used or not.

It will also be understood that the apparatus described can be modifiedin many ways within the scope of the invention. For example, althoughthe frame 14 holding the optical elements 16,24,26,30 is shown securedrigidly to the base plate 12, it may be preferred to secure theseelements to the probe body 42, directly or indirectly, so that they arefixed in relation to the optical element of the applanating head 44.

What is claimed is:
 1. A tonometer comprising: an applanating elementhaving a light-transmitting contact face engageable against the eye of asubject for projecting a light beam onto the eye and passing reflectedlight therefrom, means responsive to the reflected illumination andmeans responsive to the force of engagement of the applanating elementon the eye for obtaining data of changing values of reflectedillumination and force as the element is applied to the eye, saidcontact face having an area larger than a predetermined standard area ofengagement at which the applanating force is to be determined, theinstant of applanation of said predetermined standard area beingobtained by interpolation from a signal obtained from the reflectedillumination when the cornea is applanated over the contact face area,the instant of applanation being dependent upon the ratio of saidpredetermined standard and larger areas of applanation, in order todetermine the force measured at said instant of applanation of thepredetermined standard area.
 2. A tonometer according to claim 1comprising means for applying a correction to said determined force fora dynamic force component acting between the applanating element and theeye at said instant of applanation.
 3. A tonometer according to claim 2wherein means are provided for processing progressive measurements ofthe force and reflected illumination to derive a measure of the dynamicforce component at said instant of applanation.
 4. A tonometer accordingto claim 1 wherein the applanating element is guided for rectilineardisplacement on a resilient mounting and the applanating force isderived from a measurement of the deflection of the mounting when saidelement is applied against the eye.
 5. A tonometer according to claim 4wherein a beam splitting element is located in a path of light beamtransmission from an illumination device to the eye, the reflected lightbeam returned from the eye through the beam splitting element beingdirected onto a detecting device forming the means responsive to thereflected illumination, the beam associated with one said device passingdirectly through the beam splitter and the beam associated with theother said device being reflected by the beam splitter.
 6. A tonometeraccording to claim 5 wherein the illumination device is arranged totransmit the incident light beam along an axis extending in thedirection of displacement of the applanating element.
 7. A tonometeraccording to claim 5 comprising a monitoring light source arranged todirect light through the beam splitter onto respective front faces ofthe illumination and detection devices for reflection therefrom onto theeye of the subject, whereby the pattern of reflected light from saidsource observed by the subject will vary in dependence upon thealignment of the axis of the eye of the subject with the axis of theapplanating element.
 8. A tonometer according to claim 7 wherein saidreflecting front faces of the illumination and detection devices areeach rotationally symmetrical about an optical axis of the respectivedevice, whereby misalignment of said axes is observable as a loss ofsymmetry in the pattern of reflected illumination.
 9. A tonometeraccording to claim 2, wherein the applanating element is guided forrectilinear displacement on a resilient mounting and the applanatingforce is derived from a measurement of the deflection of the mountingwhen said element is applied against the eye.
 10. A tonometer accordingto claim 3, wherein the applanating element is guided for rectilineardisplacement on a resilient mounting and the applanating force isderived from a measurement of the deflection of the mounting when saidelement is applied against the eye.
 11. A tonometer according to claim 6comprising a monitoring light source arranged to direct light throughthe beam splitter onto respective front faces of the illumination anddetection devices for reflection therefrom onto the eye of the subject,whereby the pattern of reflected light from said source observed by thesubject will vary in dependence upon the alignment of the axis of theeye of the subject with the axis of the applanating element.
 12. Amethod according to claim 9, wherein the alignment of the optical axisof the eye of the subject with a central axis of the applanating elementis monitored by directing light from a monitoring light source in thetonometer onto respective mutually perpendicular reflecting surfaces tofollow conjugate paths to said central axis, so that the pattern of themonitoring illumination observed by the subject varies depending uponwhether there is misalignment of said axis.